Process for producing water-absorbing resin

ABSTRACT

A process for preparing a water-absorbent resin made from an α,β-unsaturated carboxylic acid as an essential monomer characterized in that the process comprises allowing a metal chelating agent to be present at any step in the process in an amount of 0.001 to 6 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid; and adding a reducing agent or an oxidizing agent thereto in an amount of 0.001 to 6 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid before initiation of drying and/or during drying of a gelated product containing a water-absorbent resin obtained by polymerization.

TECHNICAL FIELD

The present invention relates to a process for preparing awater-absorbent resin. More specifically, the present invention relatesto a process for preparing a water-absorbent resin having nodiscoloration immediately after the preparation, and having excellentdiscoloration resistance, which can be suitably used as an absorbentarticle of hygienic materials such as paper diapers and sanitarynapkins.

BACKGROUND ART

Water-absorbent resins have been widely used as absorbent articles forhygienic materials such as paper diapers and sanitary napkins, utilizingthe feature that a water-based liquid, for instance, a body fluid suchas human urine, blood or sweat is quickly absorbed in a large amount, sothat the liquid once absorbed is not released even under a load.

Conventional water-absorbent resins have some problems that thewater-absorbent resins discolor into yellow even immediately after thepreparation depending upon the preparation processes, or that the resinsare likely to discolor into yellow, brown or the like by an externalfactor such as heat or humidity when the resins are allowed to standeven if the resins are white immediately after the preparation. In thefield of the above-mentioned hygienic materials such as paper diapers,when a water-absorbent resin discolors in the absorbent article, thecommercial value as an absorbent article is drastically lowered.Therefore, under severe high-temperature, high-humidity environment suchas storehouse in the summer time, it has been desired that awater-absorbent resin does not discolor when the water-absorbent resinor absorbent article is subjected to storage for a long period of time.

As water-absorbent resins having an effect of preventing discoloration,there have been known, for instance, a high water-absorbent polymercomposition in which an organic phosphoric acid compound or a saltthereof is added to a high water-absorbent polymer (Japanese PatentLaid-Open No. Hei 5-86251); an absorbent composition comprising anacidic water-swellable crosslinked polymer, a basic water-swellablecrosslinked polymer, a discoloration preventive and/or an antioxidantand/or a boron-containing compound (Japanese Patent Laid-Open No.2000-230129); an absorbent composition comprising a water-absorbentresin, and an organic carboxylic acid and/or a salt thereof (JapanesePatent Laid-Open No. 2000-327926) and the like. However, the abovecompositions do not exhibit a sufficiently satisfactory effect ofpreventing discoloration when subjected to high-temperature,high-humidity storage for a long period of time.

Accordingly, an object of the present invention is to provide awater-absorbent resin having no discoloration immediately after thepreparation, and having suppressed discoloration even when subjected toa room temperature storage or even to a high-temperature, high-humiditystorage for a long period of time.

These and other objects of the present invention will be apparent fromthe following description.

DISCLOSURE OF INVENTION

Concretely, the present invention relates to:

-   [1] a process for preparing a water-absorbent resin made from an    α,β-unsaturated carboxylic acid as an essential monomer    characterized in that the process comprises allowing a metal    chelating agent to be present at any step in the process in an    amount of 0.001 to 6 parts by weight, based on 100 parts by weight    of the α,β-unsaturated carboxylic acid; and adding a reducing agent    or an oxidizing agent thereto in an amount of 0.001 to 6 parts by    weight, based on 100 parts by weight of the α,β-unsaturated    carboxylic acid before initiation of drying and/or during drying of    a gelated product containing a water-absorbent resin obtained by    polymerization;-   [2] a water-absorbent resin obtainable by the process of the above    [1], wherein the water-absorbent resin has Yellow Index of 12 or    less, after allowing to stand at 50° C. and 90% relative humidity    for 20 days;-   [3] an absorbent comprising a water-absorbent resin obtained by the    process of the above [1], and a hydrophilic fiber;-   [4] an absorbent article comprising the absorbent of the above [3],    wherein the absorbent is kept between a liquid-permeable sheet and a    liquid-impermeable sheet; and-   [5] a method for preventing discoloration of a water-absorbent resin    made from an α,β-unsaturated carboxylic acid as an essential    monomer, wherein the method comprises preparing the water-absorbent    resin by a process comprising allowing a metal chelating agent to be    present at any stage in the process in an amount of 0.001 to 6 parts    by weight, based on 100 parts by weight of the α,β-unsaturated    carboxylic acid; and adding a reducing agent or an oxidizing agent    thereto in an amount of 0.001 to 6 parts by weight, based on 100    parts by weight of the α,β-unsaturated carboxylic acid before    initiation of drying and/or during drying of a gelated product    containing a water-absorbent resin obtained by polymerization.

BEST MODE FOR CARRYING OUT THE INVENTION

The process for preparing a water-absorbent resin of the presentinvention has a feature in that a water-absorbent resin having nodiscoloration immediately after the preparation, and having suppresseddiscoloration even when subjected to a room temperature storage or evento a high-temperature, high-humidity storage for a long period of timecan be prepared by a process comprising allowing a metal chelating agentto be present at any step in the process in an amount of 0.001 to 6parts by weight, based on 100 parts by weight of the α,β-unsaturatedcarboxylic acid; and adding a reducing agent or an oxidizing agentthereto in an amount of 0.001 to 6 parts by weight, based on 100 partsby weight of the α,β-unsaturated carboxylic acid before initiation ofdrying and/or during drying of a gelated product containing awater-absorbent resin obtained by polymerization.

As the polymerization method for the water-absorbent resin,representative preparation processes such as reversed phase suspensionpolymerization method and aqueous solution polymerization method areusable without being limited thereto. The water-absorbent resin can beprepared by drying the water-absorbent resin obtained by theabove-mentioned polymerization method, and removing water and an organicsolvent.

At any step of preparing a water-absorbent resin, the method of allowinga metal chelating agent to be present includes (i) a method of addingthe above-mentioned chelating agent to an aqueous solution of themonomer comprising an α,β-unsaturated carboxylic acid before thepolymerization; (ii) a method of adding the above-mentioned chelatingagent to a water-containing gelated product after the polymerization;(iii) a method of adding the above-mentioned chelating agent to awater-absorbent resin during drying; (iv) a method of powder-blendingthe above-mentioned chelating agent with a water-absorbent resin afterdrying; (v) a method of adding the above-mentioned chelating agent to awater-absorbent resin dispersed in an organic solvent, and heating andremoving the solvent from the mixture; and the like.

It is preferable that the embodiment of adding a metal chelating agentis, but not particularly limited to, an embodiment of adding a solutionprepared by dissolving a liquid or powder metal chelating agent in ahydrophilic solvent such as water, or an embodiment of adding a finepowder of a metal chelating agent in a powdery state, in order that themetal chelating agent is homogeneously dispersed in the water-absorbentresin. Here, the particle size of the fine metal chelating agent powderis not particularly limited. It is preferable that 80% by weight of theentire particles have particle sizes of 100 μm or less, from theviewpoint that a satisfactory effect of preventing discoloration isobtained when the amount of coarse grain portions is smaller.

The metal chelating agent usable in the present invention includesphosphoric acid-based metal chelating agents such as pyrophosphoric acidand tripolyphosphoric acid, and salts thereof; carboxylic acid-basedmetal chelating agents such as citric acid, tartaric acid and phthalicacid, and salts thereof; aminocarboxylic acid-based metal chelatingagents such as iminodiacetic acid, hydroxyethyliminodiacetic acid,nitrilotriacetic acid, nitrilotripropionic acid,ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid,triethylenetetraminehexaacetic acid,trans-1,2-diaminocyclohexanetetraacetic acid,N,N-bis(2-hydroxyethyl)glycine, diaminopropanoltetraacetic acid,ethylenediaminedipropionic acid, hydroxyethylenediaminetriacetic acid,glycol ether diaminetetraacetic acid, diaminopropanetetraacetic acid,N,N′-bis(2-hydroxybenzyl)ethylenediamine-N,N′-diacetic acid, and1,6-hexamethylenediamine-N,N,N′,N′-tetraacetic acid, and salts thereof;and the like. Among them, phosphoric acid-based metal chelating agents,aminocarboxylic acid-based metal chelating agents and salts thereof arepreferably used, from the viewpoint of the effect of preventingdiscoloration. Especially, diethylenetriaminepentaacetic acid,triethylenetetraminehexaacetic acid,trans-1,2-diaminocyclohexanetetraacetic acid, ethylenediaminetetraaceticacid, tripolyphosphoric acid, and salts thereof are more preferablyused.

The amount of the above-mentioned metal chelating agent used is from0.001 to 6 parts by weight, preferably from 0.005 to 3 parts by weight,more preferably from 0.01 to 2 parts by weight, based on 100 parts byweight of the α,β-unsaturated carboxylic acid. When the amount of themetal chelating agent used is 0.001 parts by weight or more, asatisfactory effect for preventing discoloration can be obtained. Also,when the amount of the metal chelating agent used is 6 parts by weightor less, the effect corresponding to the amount used is obtained, and iseconomically advantageous.

It is preferable that the method of adding a reducing agent or anoxidizing agent is a method of adding the reducing agent or theoxidizing agent before initiation of drying and/or during drying of agelated product containing a water-absorbent resin obtained by thepolymerization, from the viewpoint that the procedures in thepreparation steps are simplified.

It is preferable that the embodiment of adding the reducing agent or theoxidizing agent mentioned above is, but not particularly limited to, anembodiment of adding a solution prepared by dissolving a liquid orpowder of a reducing agent or an oxidizing agent in a hydrophilicsolvent such as water, or an embodiment of adding a fine powder of areducing agent or an oxidizing agent in a powdery state, in order thatthe reducing agent or the oxidizing agent is homogeneously dispersed inthe water-absorbent resin.

In addition, the order of addition of the metal chelating agent and thereducing agent or the oxidizing agent is not particularly limited.Either one can be added first, or they may be added simultaneously. Amethod of adding a reducing agent or an oxidizing agent and thereafteradding a metal chelating agent is more preferable from the aspect of theeffect of preventing discoloration.

The reducing agent usable in the present invention includes sulfitessuch as sodium sulfite, potassium sulfite, calcium sulfite, zincsulfite, and ammonium sulfite; hydrogensulfites such as sodiumhydrogensulfite, potassium hydrogensulfite, calcium hydrogensulfite, andammonium hydrogensulfite; pyrosulfites such as sodium pyrosulfite,potassium pyrosulfite, and ammonium pyrosulfite; dithionites such assodium dithionite, potassium dithionite, ammonium dithionite, calciumdithionite, and zinc dithionite; trithionates such as potassiumtrithionate and sodium trithionate; tetrathionates such as potassiumtetrathionate and sodium tetrathionate; thiosulfates such as sodiumthiosulfate, potassium thiosulfate, and ammonium thiosulfate; nitritessuch as sodium nitrite, potassium nitrite, calcium nitrite, and zincnitrite; and the like. Among them, sulfites, hydrogensulfites,pyrosulfites, and dithionites are preferable, and sodium sulfite, sodiumhydrogensulfite, potassium pyrosulfite, and sodium dithionite are morepreferably used.

The oxidizing agent usable in the present invention includes chloritessuch as sodium chlorite; hypochlorites such as sodium hypochlorite,potassium hypochlorite, and calcium hypochlorite; peroxides such ashydrogen peroxide, sodium peroxide, potassium peroxide, potassiumpermanganate, sodium peroxoborate, benzoyl peroxide, and lauroylperoxide; and the like. Among them, hydrogen peroxide is preferablyused.

The amount of the reducing agent or the oxidizing agent mentioned aboveused is 0.001 to 6 parts by weight, preferably from 0.005 to 3 parts byweight, more preferably from 0.01 to 2 parts by weight, based on 100parts by weight of the α,β-unsaturated carboxylic acid. The amount ofthe reducing agent or the oxidizing agent used is 0.001 parts by weightor more, from the viewpoint of obtaining a water-absorbent resin havinglittle discoloration even immediately after the preparation. Inaddition, the amount of the reducing agent or the oxidizing agent usedis 6 parts by weight or less, from the viewpoint of obtaining the effectcorresponding to the amount used, thereby being economicallyadvantageous.

In the process of the present invention, the combination of the reducingagent or the oxidizing agent with the metal chelating agent to be addedis not particularly limited. The combination of a sulfite, ahydrogensulfite, a pyrosulfite, a dithionite or a peroxide with aphosphoric acid-based metal chelating agent and/or an aminocarboxylicacid-based metal chelating agent is preferable, from the viewpoint ofobtaining a water-absorbent resin having little discoloration evenimmediately after the preparation, and having suppressed discolorationeven when subjected to a room temperature storage or even to ahigh-temperature, high-humidity storage for a long period of time. Amongthem, combinations of sodium sulfite, sodium hydrogensulfite, potassiumpyrosulfite, sodium dithionite or hydrogen peroxide, with at least onemember selected from the group consisting ofdiethylenetriaminepentaacetic acid, triethylenetetraminehexaacetic acid,trans-1,2-diaminocyclohexanetetraacetic acid, ethylenediaminetetraaceticacid, tripolyphosphoric acid, and salts thereof are more preferable, andcombinations of sodium sulfite or sodium hydrogensulfite with at leastone member selected from the group consisting ofdiethylenetriaminepentaacetic acid, triethylenetetraminehexaacetic acid,trans-1,2-diaminocyclohexanetetraacetic acid, and salts thereof are evenmore preferable.

Therefore, according to the present invention, there is provided amethod for preventing discoloration of a water-absorbent resin made froman α,β-unsaturated carboxylic acid as an essential monomer, wherein themethod comprises preparing the water-absorbent resin by a processcomprising allowing a metal chelating agent to be present at any step inthe process in an amount of 0.001 to 6 parts by weight, based on 100parts by weight of the α,β-unsaturated carboxylic acid; and adding areducing agent or an oxidizing agent thereto in an amount of 0.001 to 6parts by weight, based on 100 parts by weight of the α,β-unsaturatedcarboxylic acid before initiation of drying and/or during drying of agelated product containing a water-absorbent resin obtained bypolymerization.

In a case of a water-absorbent resin obtained by a process comprisingadding a reducing agent or an oxidizing agent without adding a metalchelating agent, while Yellow Index immediately after the preparation islower than a case where the reducing agent or the oxidizing agent is notadded. Meanwhile, Yellow Index of such a resin increases with thepassage of time at room temperature or moreover under high-temperaturehigh-humidity conditions, which is the same as in the case where thereducing agent or the oxidizing agent is not added. On the other hand,the water-absorbent resin obtained by the process of the presentinvention comprising adding a reducing agent or an oxidizing agent and ametal chelating agent has very low Yellow Index immediately after thepreparation, and a subsequent increase in Yellow Index with the passageof time is remarkably suppressed.

The water-absorbent resin made from an α,β-unsaturated carboxylic acidas an essential monomer prepared in the present invention includescrosslinked products of acrylate polymers, crosslinked products ofhydrolysates of starch-acrylate graft copolymers, crosslinked productsof vinyl alcohol-acrylate copolymers, crosslinked products of maleicanhydride-grafted polyvinyl alcohol, crosslinked isobutylene-maleicanhydride copolymers, partially neutralized crosslinked products ofpolyacrylic acid, saponified products of vinyl acetate-acrylic estercopolymers, and the like. Among them, preferable are the crosslinkedproducts of acrylate polymers which are capable of absorbing a largeamount of water and retaining absorbed water in the molecule even when acertain load is applied.

The α,β-unsaturated carboxylic acid includes, for instance, acrylicacid, methacrylic acid, maleic acid, maleic anhydride, fumaric acid andthe like. These can be used alone or in admixture of two or more kinds.The α,β-unsaturated carboxylic acid may be partially neutralized with analkali metal or the like, and especially acrylic acid, methacrylic acid,and an alkali metal salt thereof, such as sodium or potassium arepreferably used.

The above-mentioned α,β-unsaturated carboxylic acid may be copolymerizedwith other monomers as occasion demands. Other monomers include, forinstance, nonionic, hydrophilic group-containing monomers such as(meth)acrylamide [“(meth)acryl-” means “acryl-” and “methacryl-;”hereinafter referred to the same], N-substituted (meth)acrylamide,2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, and polyethylene glycol(meth)acrylate; unsaturated amino group-containing monomers such asN,N-dimethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide; sulfonicacid-based monomers such as vinylsulfonic acid, styrenesulfonic acid,2-(meth)acrylamide-2-methylpropanesulfonic acid,2-(meth)acryloylethanesulfonic acid and salts thereof; and the like.

The polymerization method of the water-absorbent resin according to thereversed phase suspension polymerization method will be exemplifiedhereinbelow.

In the reversed phase suspension polymerization method, thepolymerization is carried out by using, for instance, a polymerizationinitiator in a state that an aqueous solution of the monomers isdispersed in an organic solvent in the presence of at least one of thesurfactants and the polymeric protective colloids.

It is preferable that the concentration of the monomers in theabove-mentioned aqueous solution of the monomers is from 25% by weightto a saturated concentration. In addition, the above-mentioned aqueoussolution of the monomers may be added at once or may be added in dividedportions in the polymerization reaction.

The above-mentioned organic solvent includes aliphatic hydrocarbonsolvents such as n-pentane, n-hexane, n-heptane, and ligroin; alicyclichydrocarbon solvents such as cyclopentane, methylcyclopentane,cyclohexane, and methylcyclohexane; aromatic hydrocarbon solvents suchas benzene, toluene and xylene; and the like. Among them, n-heptane andcyclohexane are preferably used.

The amount of the organic solvent used is preferably from 50 to 600parts by weight, more preferably from 100 to 550 parts by weight, basedon 100 parts by weight of the total amount of the monomers, from theviewpoints of removing heat of polymerization, thereby making it easy tocontrol the polymerization temperature.

The above-mentioned surfactant includes nonionic surfactants such assorbitan fatty acid esters, monoglycerol fatty acid esters, polyglycerolfatty acid esters, sucrose fatty acid esters, polyoxyethylenehydrogenated castor oil, polyoxyethylene laurate hydrogenated castoroil, polyoxyethylene triisostearate hydrogenated castor oil,polyoxyethylene alkylphenyl ethers, polyoxyethylene lauryl ethers, andpolyoxyethylene hexyldecyl ethers.

The above-mentioned polymeric protective colloid includes ethylcellulose, hydroxyethyl cellulose, polyethylene oxide, polyethylenemodified with maleic anhydride, polybutadiene modified with maleicanhydride, ethylene-propylene-diene terpolymers modified with maleicanhydride, and the like.

These nonionic surfactants and polymer protective colloids may be usedin admixture of two or more kinds.

The nonionic surfactant and/or the polymeric protective colloidsmentioned above can be used together with an anionic surfactant. Theanionic surfactant includes salts of fatty acids,alkylbenzenesulfonates, alkyl methyl taurates, polyoxyethylenealkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates andthe like.

The amount of the surfactant and/or the polymeric protective colloidused is preferably from 0.1 to 5 parts by weight, more preferably from0.2 to 3 parts by weight, based on 100 parts by weight of the totalamount of the monomers, from the viewpoints of having satisfactorydispersion of the aqueous solution of monomers, obtaining an effectcorresponding to the amount used, and being economically advantageous.

The above-mentioned water-absorbent resin can be prepared in aself-crosslinking reaction without using a crosslinking agent. Also, theresin can be crosslinked using an internal crosslinking agent having twoor more polymerizable unsaturated groups or two or more reactive groups.The internal crosslinking agent includes a compound having two or moreunsaturated ethylenic groups in one molecule, such asN,N′-methylenebis(meth)acrylamide, ethylene glycol di(meth)acrylate,diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate,trimethylolpropane di(meth)acrylate, trimethylolpropanetri(meth)acrylate, trimethylolpropane di(meth)allyl ether, andtriallylamine; polyglycidyl ethers, such as (poly)ethylene glycoldiglycidyl ether and glycerol triglycidyl ether; halogenated epoxycompounds, such as epichlorohydrin and epibromohydrin; and the like. Oneor more of these can be used upon considering the reactivity and watersolubility in the polymerization system. It is preferable that thecompound having two or more glycidyl groups in one molecule is used asthe internal crosslinking agent.

The amount of the internal crosslinking agent used is preferably from0.001 to 3 parts by weight, more preferably from 0.003 to 1 part byweight, even more preferably from 0.005 to 0.5 parts by weight, based on100 parts by weight of the total amount of the above-mentioned monomers,from the viewpoints of suppressing the water-soluble nature of theresulting water-absorbent resin by an appropriate crosslinking, andshowing a satisfactory water absorbency.

The polymerization initiator includes radical polymerization initiatorssuch as potassium persulfate, sodium persulfate, ammonium persulfate,benzoyl peroxide, hydrogen peroxide,1,1′-azobis(cyclohexane-1-carbonitrile),2,2′-azobis(2-methyl-butyronitrile), 2,2′-azobisisobutyronitrile,2,2′-azobis(2-amidinopropane)dihydrochloride,2-cyano-2-propylazoformamide, dimethyl 2,2′-azobis(2-methylpropionate).The radical polymerization initiator as mentioned above may be usedtogether with a sulfite or the like as a redox-system polymerizationinitiator.

It is desired that the amount of the polymerization initiator used isfrom 0.005 to 1 mol per 100 mol of the total amount of theabove-mentioned monomers, from the viewpoints of shortening the time forthe polymerization reaction, preventing an unreasonable polymerizationreaction, and facilitating the control of the polymerization reaction.

The surfactant and/or the polymeric protective colloid mentioned aboveis dissolved in an organic solvent, and the mixture of theabove-mentioned aqueous solution of the monomers, the polymerizationinitiator and the like are added to the above-mentioned organicsolution. The mixture is heated while stirring, and subjected toreversed phase suspension polymerization in a water-in-oil system. Thereaction temperature of the above-mentioned polymerization reactiondiffers depending upon the kinds of the polymerization initiator and themonomers used, or the concentration of the aqueous solution of themonomers. The reaction temperature is preferably from 20° to 110° C.,more preferably from 40° to 80° C., from the viewpoints of rapidlyprogressing the polymerization, shortening the polymerization time,being economically favorable, making the removal of the heat of reactioneasier, and smoothly carrying out the reaction. The reaction time isusually from 0.5 to 4 hours.

The resulting water-absorbent resin may be subjected tosurface-crosslinking by treating the water-absorbent resin with acrosslinking agent having two or more functional groups havingreactivity with carboxyl groups. The surface-crosslinking agent includesthose capable of reacting with carboxyl groups in the water-absorbentresin, for instance, epoxy compounds such as (poly)ethylene glycoldiglycidyl ether, (poly)propylene glycol diglycidyl ether,(poly)glycerol polyglycidyl ethers, and glycidol; halogenated epoxycompounds such as epichlorohydrin, epibromohydrin,α-methylepichlorohydrin; polyhydric alcohol compounds such as(poly)ethylene glycol, (poly)propylene glycol, (poly)glycerol, diols,pentanediols, hexanediols, trimethylolpropane, diethanolamine, andtriethanolamine; and the like. Among them, the epoxy compound is morepreferably used. These surface-crosslinking agents can be used alone orin combination of two or more kinds.

The amount of the surface-crosslinking agent used is preferably from0.01 to 5 parts by weight, more preferably from 0.02 to 4 parts byweight, even more preferably from 0.03 to 3 parts by weight, based on100 parts by weight of the total amount of the above-mentioned monomer,from the viewpoints of obtaining satisfactory gel strength and waterabsorbency during water absorption of the water-absorbent resin.

The method of adding the above-mentioned surface-crosslinking agent isnot particularly limited. The method includes, for instance, a methodcomprising adding the above-mentioned surface-crosslinking agent to awater-absorbent resin dispersed in an organic solvent; a methodcomprising spraying the above-mentioned surface-crosslinking agent to awater-absorbent resin with a spray or the like while stirring; and thelike. The timing of the addition for the surface-crosslinking agentincludes the stage of the water-containing gelated product after thepolymerization; the stage of the water-containing particles duringdrying; the stage after drying; and the like. Among them, a methodcomprising adding the surface-crosslinking agent to a water-absorbentresin dispersed in an organic solvent at the stage of thewater-containing gelated product after the polymerization, and a methodcomprising spraying with a spray or the like at the stage of thewater-containing particles during drying are preferable.

The embodiment of adding the surface-crosslinking agent is notparticularly limited. In order that the surface-crosslinking agent ishomogeneously added to the water-absorbent resin, it is preferable thatthe surface-crosslinking agent is dissolved in a hydrophilic solventsuch as water before addition.

The water-absorbent resin can be prepared by drying the water-absorbentresin obtained by the polymerization method mentioned above, andremoving water and an organic solvent therefrom. The resultingwater-absorbent resin may be classified with sieves or the like asoccasion demands.

In a case where the resulting water-absorbent resin is used in the formof an absorbent and an absorbent article, Yellow Index is preferably 12or less, more preferably 10 or less, after the water-absorbent resin isallowed to stand at 50° C. under a 90% relative humidity for 20 days.Furthermore, the difference between Yellow Index of the water-absorbentresin after being allowed to stand at 50° C. under 90% relative humidityfor 20 days and Yellow Index of the water-absorbent resin before beingallowed to stand is preferably 2 or less, more preferably 1 or less.Here, Yellow Index in the present invention refers to a value determinedby the measurement method described later.

In the present invention, an absorbent and an absorbent article can beproduced using the water-absorbent resin obtained by the above-mentionedpreparation process. The absorbent comprises a water-absorbent resin anda hydrophilic fiber. The usable hydrophilic fiber includes cellulosefibers, artificial cellulose fibers and the like, without beingparticularly limited thereto. The hydrophilic fiber may contain asynthetic fiber. The proportion of the water-absorbent resin and thehydrophilic fiber is not particularly limited. In addition, theconstitution of the absorbent includes, for instance, a mixing structurein which a water-absorbent resin and a hydrophilic fiber arehomogeneously blended; a sandwich structure in which a water-absorbentresin is spread between layered hydrophilic fibers; and the like,without being particularly limited thereto.

The absorbent article can be produced by, for instance, putting theabove-mentioned absorbent between a liquid-permeable sheet and aliquid-impermeable sheet. The liquid-permeable sheet includes poroussheets or nonwoven fabrics made of polyethylene, polypropylene, or apolyester, and the like, and the liquid-impermeable sheet includessynthetic resin films made of polyethylene, polypropylene or polyvinylchloride, and the like, without being limited thereto.

The absorbent article using the water-absorbent resin of the presentinvention is preferably, for instance, a hygienic material such as paperdiaper, sanitary napkin or incontinence pad; a urine-absorbing materialfor pets, without being particularly limited thereto. Besides, there canbe used, for instance, for materials for civil engineering andconstruction such as packing materials; food freshness retainingmaterials such as drip absorbents and keeping cool agents; horticulturalarticles such as water-retaining materials for soils; and the like.

The present invention will be explained hereinbelow by means of Examplesand Comparative Examples, without intending to limit the presentinvention only to these Examples.

Example 1

Five-hundred milliliters of n-heptane was added to a 1000-ml five-neckedcylindrical round bottomed flask equipped with a stirrer, a refluxcondenser, a dropping funnel, a thermometer and a nitrogen gas inlettube. Thereto was added 0.92 g of sucrose fatty acid ester (surfactant:S-370, manufactured by MITSUBISHI CHEMICAL CORPORATION) having an HLB of3.0 and dispersed. The temperature of the dispersion was raised todissolve the surfactant, and thereafter cooled to 55° C.

Separately from the above, 92 g of a 80% by weight aqueous solution ofacrylic acid was added to a 500-ml Erlenmeyer flask. Thereto was addeddropwise 102.2 g of a 30% by weight aqueous sodium hydroxide solutionwhile externally cooling, to neutralize 75% by mol of acrylic acid, togive a partially neutralized salt of acrylic acid. Further, 50.2 g ofwater, 0.11 g of a polymerization initiator potassium persulfate, and9.2 mg of a crosslinking agent ethylene glycol diglycidyl ether wereadded thereto, to give an aqueous solution of a monomer for a first steppolymerization.

The entire amount of this aqueous solution of the monomer for a firststep polymerization was added to the above-mentioned five-neckedcylindrical round bottomed flask under stirring and dispersed. After theinternal of the system was sufficiently replaced with nitrogen, thetemperature of the mixture was raised, and the polymerization reactionwas carried out for 1 hour while keeping its bath temperature at 70° C.Thereafter, the polymerization slurry was cooled to room temperature.

Further, 119.1 g of a 80% by weight aqueous solution of acrylic acid wasadded to a separate 500-ml Erlenmeyer flask. Thereto was added dropwise132.2 g of a 30% by weight aqueous sodium hydroxide solution whilecooling, to neutralize 75% by mol of acrylic acid. Further, 27.4 g ofwater, 0.14 g of potassium persulfate, and 35.7 mg of ethylene glycoldiglycidyl ether were added thereto, to give an aqueous solution of amonomer for a second step polymerization. The aqueous solution wascooled in an ice water bath.

The entire amount of this aqueous solution of the monomer for a secondstep polymerization was added to the above-mentioned polymerizationslurry. After the internal of the system was again sufficiently replacedwith nitrogen, the temperature of the mixture was raised, and thesecond-step polymerization reaction was carried out for 2 hours whilekeeping its bath temperature at 70° C. After the termination of thepolymerization, 3.52 g of a 3% by weight aqueous sodium sulfite solutionwas added to a water-containing gelated product dispersed in n-heptane,and the mixture was stirred for 30 minutes. Thereafter, 5.28 g of a 40%by weight aqueous solution of pentasodium diethylenetriaminepentaacetatewas added thereto under stirring. Subsequently, water of thewater-containing gelated product was removed to the external of thesystem by azeotropic dehydration. To the resulting gelated product wasadded 8.44 g of a 2% by weight aqueous solution of ethylene glycoldiglycidyl ether, and water and n-heptane were further removed from themixture by distillation, and the residue was dried, to give 215.5 g of awater-absorbent resin.

Example 2

The same procedures as in Example 1 were carried out except that theamount of the 40% by weight aqueous solution of pentasodiumdiethylenetriaminepentaacetate in Example 1 was changed to 0.528 g, togive 213.1 g of a water-absorbent resin.

Example 3

The same procedures as in Example 1 were carried out except that theamount of the 40% by weight aqueous solution of pentasodiumdiethylenetriaminepentaacetate in Example 1 was changed to 0.264 g, togive 212.9 g of a water-absorbent resin.

Example 4

Five-hundred milliliters of n-heptane was added to a 1000-ml five-neckedcylindrical round bottomed flask equipped with a stirrer, a refluxcondenser, a dropping funnel, a thermometer and a nitrogen gas inlettube. Thereto was added 1.38 g of decaglycerol pentastearate(surfactant: SUNSOFT Q-185S, manufactured by Taiyo Kagaku Co., Ltd.)having an HLB of 5.0 and dispersed. The temperature of the dispersionwas raised to dissolve the surfactant, and thereafter cooled to 55° C.

Separately from the above, 92 g of a 80% by weight aqueous solution ofacrylic acid was added to a 500-ml Erlenmeyer flask. Thereto was addeddropwise 102.2 g of a 30% by weight aqueous sodium hydroxide solutionwhile externally cooling, to neutralize 75% by mol of acrylic acid, togive a partially neutralized salt of acrylic acid. Further, 50.2 g ofwater, 0.11 g of a polymerization initiator potassium persulfate, and18.4 mg of a crosslinking agent ethylene glycol diglycidyl ether wereadded thereto, to give an aqueous solution of a monomer for thepolymerization.

The entire amount of this aqueous solution of the monomer for thepolymerization was added to the above-mentioned five-necked cylindricalround bottomed flask under stirring and dispersed. After the internal ofthe system was sufficiently replaced with nitrogen, the temperature ofthe mixture was raised, and the polymerization reaction was carried outfor 1 hour while keeping its bath temperature at 70° C. After thetermination of the polymerization, 3.07 g of a 3% by weight aqueoussodium hydrogensulfite solution was added thereto, and the mixture wasstirred for 30 minutes. Thereafter, 0.66 g of a 14% by weight aqueoussolution of tetrasodium trans-1,2-diaminocyclohexanetetraacetate wasadded to a water-containing gelated product dispersed in n-heptane understirring. Subsequently, water of the water-containing gelated productwas removed to the external of the system by azeotropic dehydration. Tothe resulting gelated product was added 4.14 g of a 2% by weight aqueoussolution of ethylene glycol diglycidyl ether, and water and n-heptanewere further removed from the mixture by distillation, and the residuewas dried, to give 92.3 g of a water-absorbent resin.

Example 5

The same procedures as in Example 4 were carried out except that 3.07 gof the 3% by weight aqueous sodium hydrogensulfite solution in Example 4was changed to 1.53 g of a 3% by weight aqueous sodium dithionitesolution, and that 0.66 g of the 14% by weight aqueous solution oftetrasodium trans-1,2-diaminocyclohexanetetraacetate in Example 4 waschanged to 0.24 g of a 38% by weight aqueous solution of tetrasodiumethylenediaminetetraacetate, to give 92.1 g of a water-absorbent resin.

Example 6

Five-hundred milliliters of n-heptane was added to a 1000-ml five-neckedcylindrical round bottomed flask equipped with a stirrer, a refluxcondenser, a dropping funnel, a thermometer and a nitrogen gas inlettube. Thereto was added 0.92 g of sucrose fatty acid ester (surfactant:S-370, manufactured by MITSUBISHI CHEMICAL CORPORATION) having an HLB of3.0 and dispersed. The temperature of the dispersion was raised todissolve the surfactant, and thereafter cooled to 55° C.

Separately from the above, 92 g of a 80% by weight aqueous solution ofacrylic acid was added to a 500-ml Erlenmeyer flask. Thereto was addeddropwise 102.2 g of a 30% by weight aqueous sodium hydroxide solutionwhile externally cooling, to neutralize 75% by mol of acrylic acid, togive a partially neutralized salt of acrylic acid. Further, 50.2 g ofwater, 0.11 g of a polymerization initiator potassium persulfate, and9.2 mg of a crosslinking agent ethylene glycol diglycidyl ether wereadded thereto, to give an aqueous solution of monomers for thepolymerization.

The entire amount of this aqueous solution of the monomers for thepolymerization was added to the above-mentioned five-necked cylindricalround bottomed flask under stirring and dispersed. After the internal ofthe system was sufficiently replaced with nitrogen, the temperature ofthe mixture was raised, and the polymerization reaction was carried outfor 1 hour while keeping its bath temperature at 70° C. After thetermination of the polymerization, water of the water-containing gelatedproduct was removed to the external of the system by azeotropicdehydration. To the resulting gelated product were added 3.07 g of a 3%by weight aqueous solution of potassium pyrosulfite and 4.14 g of a 2%by weight aqueous solution of ethylene glycol diglycidyl ether. Waterwas removed again together with n-heptane by distillation, and theresidue was dried. Thereafter, 0.46 g of a powdertriethylenetetraminehexaacetic acid (87% by weight of the entireparticles thereof having sizes of 100 μm or less) was added to theresidue under stirring, and the mixture was further dried, to give 93.5g of a water-absorbent resin.

Example 7

The same procedures as in Example 6 were carried out except that 3.07 gof the 3% by weight aqueous solution of potassium pyrosulfite in Example6 was changed to 1.53 g of a 3% by weight aqueous sodium sulfitesolution, and that triethylenetetraminehexaacetic acid was not added, togive 92.2 g of a water-absorbent resin. The entire amount of theabove-mentioned water-absorbent resin and 0.92 g of a powder disodiumdiethylenetriaminepentaacetate (85% by weight of the entire particleshaving particle sizes of 100 μm or less) were added into a polyethylenebag, and the mixture was sufficiently mixed, to give 93.1 g of awater-absorbent resin.

Example 8

Five-hundred milliliters of n-heptane was added to a 1000-ml five-neckedcylindrical round bottomed flask equipped with a stirrer, a refluxcondenser, a dropping funnel, a thermometer and a nitrogen gas inlettube. Thereto was added 0.92 g of sorbitan monostearate (surfactant:Nonion SP-60R, manufactured by NOF Corporation) having an HLB of 4.7 anddispersed. The temperature of the dispersion was raised to dissolve thesurfactant, and thereafter cooled to 50° C.

Separately from the above, 92 g of a 80% by weight aqueous solution ofacrylic acid was added to a 500-ml Erlenmeyer flask. Thereto was addeddropwise 102.2 g of a 30% by weight aqueous sodium hydroxide solutionwhile externally cooling, to neutralize 75% by mol of acrylic acid, togive a partially neutralized salt of acrylic acid. Further, 20.8 g ofwater, 0.11 g of a polymerization initiator potassium persulfate, and23.0 mg of a crosslinking agent N,N′-methylenebisacrylamide were addedthereto. To the mixture was added 0.092 g of sodium tripolyphosphate, togive an aqueous solution of a monomer for the polymerization.

The entire amount of this aqueous solution of the monomer for thepolymerization was added to the above-mentioned five-necked cylindricalround bottomed flask under stirring and dispersed. After the internal ofthe system was sufficiently replaced with nitrogen, the temperature ofthe mixture was raised, and the polymerization reaction was carried outfor 2 hours while keeping its bath temperature at 70° C. After thetermination of the polymerization, 0.153 g of a 3% by weight aqueoushydrogen peroxide solution was added thereto, and the mixture wasstirred for 30 minutes. Subsequently, water of the water-containinggelated product was removed to the external of the system by azeotropicdehydration. To the resulting gelated product was added 4.60 g of a 2%by weight aqueous solution of ethylene glycol diglycidyl ether, andwater and n-heptane were further removed from the mixture bydistillation, and the residue was dried, to give 94.2 g of awater-absorbent resin.

Comparative Example 1

The same procedures as in Example 1 were carried out except that theaqueous sodium sulfite solution and the aqueous solution of pentasodiumdiethylenetriaminepentaacetate in Example 1 were not used, to give 214.5g of a water-absorbent resin.

Comparative Example 2

The same procedures as in Example 1 were carried out except that theaqueous solution of pentasodium diethylenetriaminepentaacetate inExample 1 was not used, to give 215.1 g of a water-absorbent resin.

Comparative Example 3

The same procedures as in Example 1 were carried out except that theaqueous sodium sulfite solution in Example 1 was not used, to give 214.7g of a water-absorbent resin.

Comparative Example 4

The same procedures as in Example 6 were carried out except thattriethylenetetraminehexaacetic acid in Example 6 was not used, to give92.9 g of a water-absorbent resin.

Comparative Example 5

The same procedures as in Example 8 were carried out except that sodiumtripolyphosphate in Example 8 was not used, to give 93.8 g of awater-absorbent resin.

Comparative Example 6

The same procedures as in Example 8 were carried out except thathydrogen peroxide in Example 8 was not used, to give 94.0 g of awater-absorbent resin.

The discoloration test of the water-absorbent resins obtained inExamples and Comparative Examples mentioned above, and the discolorationtest of the absorbent articles prepared using the water-absorbent resinswere carried out in accordance with the following methods.

(1) Discoloration Test of Water-Absorbent Resins

Into a polypropylene vessel having an inner diameter of 3 cm and a depthof 1 cm was evenly placed 2.0 g of a water-absorbent resin. This vesselwas allowed to stand for 20 days in a bench-type thermohygrostat set ata temperature of 50°±2° C. and relative humidity of 90±2% RH. Afterallowing to stand, the vessel was taken out from the thermohygrostat,and allowed to stand for some time to cool to room temperature. Theentire amount of the water-absorbent resin in the vessel was added to aglass measuring vessel having an inner diameter of 3 cm, and YellowIndex of the water-absorbent resin was determined with a doublebeam-type spectrocolorimeter Z-1001DP (manufactured by Nippon DenshokuKogyo Industries, Co., Ltd.), in which X, Y and Z, tristimulus values ofthe spectrocolorimeter are corrected with a standard white board. YellowIndex was calculated by the following equations from X, Y and Z(tristimulus values) of the resulting water-absorbent resin. Similarly,Yellow Index of the water-absorbent resin before the test of allowing tostand in the bench-type thermohygrostat for 20 days was obtained. Theabove-mentioned determinations were taken thrice, and an averaged valuewas obtained.Yellow Index=100(1.28X−1.06Z)/Y(2) Discoloration Test of Absorbent Article

A blend of 5 g of a water-absorbent resin and 5 g of a disintegratedpulp was formed on a tissue paper of 20×12 cm by air-blow molding. Atissue paper of the same size was overlaid thereto, and thereafter a 145kPa load was applied thereto for 30 seconds, to give an absorbent. Thisabsorbent was inserted between a polyethylene air-through-type nonwovenfabric having a weighing capacity of 20 g/cm² placed on top and aliquid-impermeable polyethylene sheet at bottom, to give an absorbentarticle. This absorbent article was allowed to stand for 50 days in abench-type thermohygrostat set at a temperature of 50°±2° C. andrelative humidity of 90±2% RH. After allowing to stand, thediscoloration of the water-absorbent resin in the absorbent article wasvisually observed, and evaluated according to the following criteria.

[Evaluation Criteria]

-   A: Inner water-absorbent resin does not discolor when observed while    removing the nonwoven fabric and unweaving the absorbent.-   B: Although discoloration due to the water-absorbent resin is not    found when observed from the top of the nonwoven fabric,    discoloration is found in a part of the water-absorbent resin when    the nonwoven fabric is removed and the absorbent is unweaved.-   C: Discoloration due to the water-absorbent resin is found when    observed from the top of the nonwoven fabric.

The metal chelating agent, and the oxidizing agent or reducing agentused in Examples and Comparative Examples mentioned above are shown inTable 1. The amount inside the parenthesis in Table 1 is an amount basedon 100 parts by weight of acrylic acid.

TABLE 1 Reducing Agent or Metal Chelating Agent Oxidizing Agent (partsby weight) (parts by weight) Ex. 1 Pentasodium Diethylenetriamine-Sodium Sulfite (0.0625) pentaacetate (1.25) Ex. 2 PentasodiumDiethylenetriamine- Sodium Sulfite (0.0625) pentaacetate (0.125) Ex. 3Pentasodium Diethylenetriamine- Sodium Sulfite (0.0625) pentaacetate(0.0625) Ex. 4 Tetrasodium trans-1,2-Diaminocyclo- SodiumHydrogensulfite hexanetetraacetate (0.125) (0.125) Ex. 5 TetrasodiumEthylenediaminete- Sodium Dithionite traacetate (0.124) (0.0624) Ex. 6Triethylenetetramine- Potassium Pyrosulfite hexaacetic acid (0.625)(0.125) Ex. 7 Disodium Diethylenetriamine- Sodium Sulfite (0.0624)pentaacetate (1.25) Ex. 8 Sodium Tripolyphosphate (0.125) HydrogenPeroxide (0.00624) Comp. (Not Added) (Not Added) Ex. 1 Comp. (Not Added)Sodium Sulfite (0.0625) Ex. 2 Comp. Pentasodium Diethylenetriamine- (NotAdded) Ex. 3 pentaacetate (1.25) Comp. (Not Added) Potassium PyrosulfiteEx. 4 (0.125) Comp. (Not Added) Hydrogen Peroxide Ex. 5 (0.00624) Comp.Sodium Tripolyphosphate (0.125) (Not Added) Ex. 6

In addition, the results for the discoloration test of thewater-absorbent resins obtained in Examples and Comparative Examplesmentioned above, and the discoloration test of the absorbent articlesprepared using the water-absorbent resins are shown in Table 2.

TABLE 2 Results for Discoloration Test Water-Absorbent Resin AbsorbentYellow Index Yellow Index Difference in Article Before Test After TestYellow Index Visual (YI_(a)) (YI_(b)) (YI_(b)−YI_(a)) Evaluation Ex. 16.8 7.0 0.2 A Ex. 2 7.1 7.6 0.5 A Ex. 3 7.4 8.2 0.8 A Ex. 4 7.2 7.8 0.6A Ex. 5 7.3 8.7 1.4 A Ex. 6 6.9 7.3 0.4 A Ex. 7 7.2 8.1 0.9 A Ex. 8 7.59.5 2.0 A Comp. 10.4 21.1 10.7 C Ex. 1 Comp. 7.7 24.0 16.3 C Ex. 2 Comp.9.7 13.3 3.6 B Ex. 3 Comp. 6.8 25.1 18.3 C Ex. 4 Comp. 7.5 26.3 18.8 CEx. 5 Comp. 10.2 17.3 7.1 C Ex. 6

It can be seen from the results for the discoloration test of thewater-absorbent resin of Table 2 that the water-absorbent resinsobtained in Examples have low Yellow Index, and little change in YellowIndex even when subjected to high-temperature, high-humidity storage, sothat the discoloration is suppressed. The water-absorbent resins towhich the reducing agent or oxidizing agent is not added obtained inComparative Examples 1, 3 and 6 have high Yellow Index before the testas compared to the water-absorbent resin obtained in Examples. Thewater-absorbent resins to which the reducing agent or oxidizing agent isadded but not a metal chelating agent obtained in Comparative Examples2, 4 and 5 have low Yellow Index immediately after the preparation, buthave a large change in Yellow Index when subjected to high-temperature,high-humidity storage, and more likely to discolor. Further, it can beseen from the results for the discoloration test by visual observationof the absorbent articles that the discoloration in the water-absorbentresin after being subjecting to a high-temperature, high-humiditystorage is not found in the absorbent articles obtained in Examples,while the discoloration in the water-absorbent resin after beingsubjecting to high-temperature, high-humidity storage is found in theabsorbent articles obtained in Comparative Examples.

INDUSTRIAL APPLICABILITY

According to the present invention, there can be prepared awater-absorbent resin having no discoloration immediately after thepreparation, and having suppressed discoloration even when subjected toa room temperature storage or high-temperature, high-humidity storagefor a long period of time. Therefore, an absorbent and an absorbentarticle using the water-absorbent resin obtained by the process of thepresent invention maintain excellent commercial values.

The present invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A process for preparing a water-absorbent resin, wherein said processcomprises: a) polymerizing to completion an α,β-unsaturated carboxylicacid monomer to produce a polymerized water-containing gelated product;b) adding a metal chelating agent at any step in the preparation of thewater-absorbent resin, wherein said metal chelating agent is added in anamount of 0.001 to 6 parts by weight, based on 100 parts by weight ofthe α,β-unsaturated carboxylic acid; c) adding a reducing agent or anoxidizing agent to the polymerized water-containing gelated product inan amount of 0.001 to 2 parts by weight, based on 100 parts by weight ofthe α,β-unsaturated carboxylic acid; and d) drying the gelated productthereby yielding a polymerized water-absorbent resin having greaterdiscoloration resistance than a polymerized water-absorbent resin havingno reducing or oxidizing agent and no metal chelating agent addedthereto.
 2. The process for preparing a water-absorbent resin accordingto claim 1, wherein the reducing agent is a sulfite, a hydrogensulfite,a dithionite or a pyrosulfite.
 3. The process for preparing awater-absorbent resin according to claim 1, wherein the oxidizing agentis hydrogen peroxide.
 4. The process for preparing a water-absorbentresin according to claim 1, wherein the metal chelating agent is atleast one member selected from the group consisting ofdiethylenetriaminepentaacetic acid, triethylenetetraminehexaacetic acid,trans-1,2-diaminocyclohexanetetraacetic acid, ethylenediaminetetraaceticacid, tripolyphosphoric acid, and salts thereof.
 5. A water-absorbentresin obtained by the process of any one of claims 1 to 4, wherein thewater-absorbent resin has Yellow Index of 12 or less, after allowing tostand at 50° C. and 90% relative humidity for 20 days.
 6. An absorbentcomprising a water-absorbent resin obtained by the process of any one ofclaims 1 to 4, and a hydrophilic fiber.
 7. An absorbent articlecomprising the absorbent of claim 6, wherein the absorbent is keptbetween a liquid-permeable sheet and a liquid-impermeable sheet.
 8. Aprocess for preparing a water-absorbent resin, wherein said processcomprises, in the following order: a) polymerizing to completion anα,β-unsaturated carboxylic acid monomer to produce a polymerizedwater-containing gelated product; b) adding a metal chelating agent atany step in the preparation of the water-absorbent resin, wherein saidmetal chelating agent is added in an amount of 0.001 to 6 parts byweight, based on 100 parts by weight of the α,β-unsaturated carboxylicacid; c) adding an oxidizing agent to the polymerized water-containinggelated product in an amount of 0.001 to 2 parts by weight, based on 100parts by weight of the α,β-unsaturated carboxylic acid; and d) dryingthe gelated product thereby yielding a polymerized water-absorbent resinhaving greater discoloration resistance than a polymerizedwater-absorbent resin having no oxidizing agent and no metal chelatingagent added thereto.
 9. The process for preparing a water-absorbentresin according to claim 1, wherein the amount of the reducing agent oroxidizing agent added in step c) is 0.01 to 2 parts by weight, based on100 parts by weight of the α,β-unsaturated carboxylic acid.
 10. Theprocess for preparing a water-absorbent resin according to claim 8,wherein the amount of the oxidizing agent added in step c) is 0.01 to 2parts by weight, based on 100 parts by weight of the α,β-unsaturatedcarboxylic acid.